In the oil and gas industry, bores are drilled from surface to access subsurface hydrocarbon-bearing formations. In such a drilling operation, a drill bit is mounted on the end of a long “string” of pipe sections, and may be rotated from surface or by a motor located adjacent the drill bit. Drilling fluid or “mud” is pumped from surface down through the tubular string, to exit the drill bit via jetting nozzles. The drilling fluid then passes back to surface via the annulus between the drill pipe string and the bore wall. The drilling fluid serves a number of purposes, one being to carry drill cuttings away from the drill bit and then up through the annulus to surface. For a number of reasons, and particularly in highly, deviated or extended reach wells, drill cuttings will sometimes gather in the annulus, restricting the flow of drilling fluid to surface and causing numerous other problems.
One method of clearing drill cuttings from the annulus is to provide one or more bypass tools in the drill string. These tools allow drilling fluid to flow directly into the annulus from an intermediate part of the drill pipe string, without having to pass through the drill bit and other tools normally located towards the end of a drill string, which tools collectively form a bottom hole assembly (BHA). As a result, the fluid entering the annulus via the bypass tool is at higher velocity and is more effective at carrying and clearing the drill cuttings from the annulus. Bypass tools may also be used in other circumstances where it is desirable or necessary to circulate or supply fluid to the annulus without passing the fluid through the BHA.
There have been many proposals to provide fluid actuated bypass tools relying on a differential pressure force created by the flow of fluid through the tool to open the tool, usually by translating a sleeve to permit flow through a number of side or flow ports in the wall of the tool body. In the late 1970's Emery (U.S. Pat. No. 4,298,077) proposed a bypass valve with a flow responsive differential pressure member, a biasing spring and a controlling cam arrangement. Since then there have been many tools proposed along similar lines. However, none of these tools have had widespread general use due to the tools being unreliable in many situations, although there are a few specific applications where some of the tools do work well.
In the late 1980's Lee (U.S. Pat. No. 5,499,687) proposed a tool where the string bore could be completely blocked off to actuate the tool, by dropping a nylon ball from surface to land in a seat and create a piston which is pushed down by fluid pressure above the ball to open the ports against a spring. This situation could then be reversed by dropping a second smaller steel ball which would block off the port allowing the first ball to be squeezed through its seat and the ports to be closed again. This form of tool may be necessary where it is desired to circulate materials, for example lost-circulation material (LCM), that might damage the BHA, or the BHA includes flow actuated tools which it is preferred to have inoperative during the bypass operation. Lee's tool can also be used to assist in carrying and clearing the cuttings from the annulus. Consequently this tool is prolifically used worldwide in a wide array of well bore applications.
In the mid 1990's Davy et al (WO 9630621), Pia et al (U.S. Pat. No. 5,890,540) and MacDonald (U.S. Pat. No. 5,901,796) proposed flow activated bypass tools which can selectively bypass and seal off the through bore below the bypass ports. However, the added complication of sealing off the through bore has made this form of flow activated tool even more technically challenging, and such tools are still not commercially available.
Other than tools adapted to be completely closed by a ball or the like, such as described by Lee, there are two main mechanisms available for creating a flow activated differential pressure in a tool. The first is by providing a fixed flow restriction, usually a sleeve defining a nozzle, inside the tool. The nozzle creates a distinct pressure drop due to the fluid being forced through the narrow throat of the nozzle, and this pressure acts over the cross sectional area of the sleeve and creates a force in the same direction as the flow. The disadvantages of this method are that the presence of the nozzle creates an additional pressure drop in the string and also the nozzle creates a bore restriction within the string, both of which are undesirable. Bailey et al (U.S. Pat. No. 5,443,129) and Hennig et al (U.S. Pat. No. 5,609,178) described tools where fixed flow restrictions in the form of nozzles or rings are used to power bypass tools.
The other mechanism for creating a flow activated differential pressure is to utilise the differential pressure between the inside and the outside of the pipe. This differential pressure acts via a differential piston, which is a common feature in many downhole tools. Such a piston allows the lower external pressure to act on part of the area of the sliding sleeve and the higher internal pressure to act on an opposing part of the sleeve, so creating a pressure differential force that may be utilised to move a valve sleeve. A differential piston can be configured to move in either direction relative to the direction of flow. This mechanism has neither of the major drawbacks of the nozzle method in that it can provide very significant flow related forces without inducing losses in the flowing fluid and without restricting the tool bore.
However, there a number of difficulties and uncertainties associated with the use of differential pistons, as discussed below. In general terms, the pressure at any point in the pipe or annulus is equal to the sum of all of the pressure losses created downstream of that point by the fluid flowing through the remainder of the fluid circulation path; this is known as the backpressure. Different parts of the string will create different degrees of pressure loss, but every element of the fluid flow path will contribute some pressure loss: each length of pipe, each narrowing at a screwed connection, and every piece of equipment that is part of the drill string will create a pressure loss. In general, where the flow area is small the pressure losses will be greatest. Each of these pressure losses will increase exponentially with the flow rate, such that doubling the flow rate quadruples the pressure loss.
Thus, it can be seen that the magnitude of the opening force provided by a differential piston is largely dependent on the geometry of the pipe and hole below the tool which incorporates the piston, and so will be different for every well. However, in addition, and far more significantly, the force created by a differential piston-actuated bypass tool will only exist when the flow ports are closed. The instant the ports open, flow will divert through the ports, and consequently the flow rate will reduce through the string below the tool. If, for example, the flow is split with ¼ continuing to the bit, the differential pressure force produced by the piston will suddenly be 1/16th of the force produced the instant before, when the ports were closed. Thus, the port opening force will suddenly be 1/16th of the force required to overcome the spring and open the port: opening the side ports relieves the pressure that powers the movement of the sleeve to open the port, so the sleeve immediately moves to close the ports. Directly the sleeve has closed the ports the differential pressure force will be restored and the sleeve will be moved to open the ports, and so on. However, if the tool is provided with any form of cycling control system the sleeve may shuttle back and forward until stabilising. Clearly, if the sleeve stabilises in the closed position the tool cannot be used as a bypass tool. If the sleeve shuttles to a stable position in which the sleeve is locked open it will not then be possible to close the ports, as there is very little differential pressure available to overcome the spring force and release the sleeve.
Thus, despite the attendant disadvantages, the most effective flow actuated bypass tools tend to include nozzles or other flow restrictions to create a fluid-flow related opening force: see, for example, applicant's WO 01/06086, the disclosure of which is incorporated herein by reference. However, particularly in circumstances where there is an elevated pressure differential between the tool interior and the annulus, such bypass tools often prove difficult to open. Furthermore, in circumstances where it is only possible to achieve a restricted fluid circulation flow rate, and thus a restricted fluid pressure force across the nozzle, it may be difficult to achieve the force necessary to open the bypass tool.
Even where a bypass tool is successfully opened in a high pressure differential situation, there is also often a problem relating to the initial flow of fluid through the tool flow ports: as the tool opens, the high differential pressure will induce a high velocity flow, which may result in erosion of areas of the tool, and the high velocity flow may also wash out the seals adjacent the flow ports, one of which must pass across the flow ports as the tool is opened. In particular, parts of the seals may be displaced and pushed or sucked through the flow ports, such that when the tool subsequently closes the seals are guillotined, rendering the tool useless.
Thus, although flow-operated bypass tools are currently being successfully used by many operators, the wider use of such tools is restricted by a number of limiting operating parameters, primarily differential pressure and available flow rate, and operation beyond these boundaries tends to have a negative effect on tool reliability and dependability. Accordingly, it is among the objectives of embodiments of the present invention to provide bypass tools capable of operating reliably over a wide range of hydrostatic pressures, differential pressures and flow rates. Also, it is an object of an embodiment of the invention to provide a bypass tool which can block flow to the through bore below the ports while the ports are open.